1. Field of the Art
The present invention relates to a process for preparing ethylene polymers having a moderately broad molecular weight distribution.
2. Background Art
Ethylene polymers have different applications depending on their molecular weight distributions, and those having a broad molecular weight distribution is often used for hollow molded articles such as pipes or the like, those having a moderately broad molecular weight distribution for products such as fibers or tapes and those having a narrow molecular weight distribution for injection-molded articles such as bottle caps or buckets.
Hitherto, a solid catalyst component which is known to have a high activity and comprises a magnesium halide and a titanium halide gives generally an ethylene polymer having a narrow molecular weight distribution, and the solid catalyst component may thus be suited for the production of injection molded articles such as bottle caps or buckets but not for the production of the articles for the other applications described above.
In order to expand applications of ethylene polymers, the development of a catalyst which affords a polymer having a wide molecular weight distribution is required, and many inventions with use of a variety of transition metal compounds or with a method for support a catalyst component on an inorganic carrier have been described in, for example, Japanese Patent Publication Nos. 37037/1977, 8588/1978, 8006/1980, 45247/1982, 13084/1983 and 58364/1987.
The present inventors have already proposed a process for preparing solid catalyst components particularly suited for slurry polymerization or vapor phase polymerization wherein a component which is the same component (A-1) of the present invention which will hereinbelow be described in more detail is treated with (1) a silicon halide, (2) a titanium halide or (3) a titanium halide and a hydrogenpolysiloxane (e.g. Japanese Patent Laid-Open Publication Nos. 127706/1983, 285203/1986, 285204/1986, 285205/1986, 180612/1982, 5309/1983 and 5311/1983). These catalysts, notwithstanding their usefulness as they were, resulted in polymers having narrow molecular weight distributions or were still required to be improved in the standpoint of their catalyst activities.
They have also proposed the catalysts which are obtained by treating a component which is the same component (A-1) of the present invention with (1) an aluminium halide, (2) an aluminium halide and a titanium or silicon halide or (3) an organoaluminium compound or a hydrogenpolysiloxane and an aluminium halide (see e.g. Japanese Patent Laid-Open Publication Nos. 12903/1984 and 43007/1984). These catalysts may, however, have a problem to an industrial practice thereof because of the difficulty of transportation due to the handling of the solid aluminium halides and the corrosion of the apparatuses due to the aluminum halides, although they have relatively high activities and afford products having a wide molecular weight distribution.
Furthermore, they have also proposed the catalysts obtained by treating a component which is the same component (A-1) of the present invention with a titanium or silicon halide (e.g. Japanese Patent Laid-Open Publication No. 225104/1983). These catalysts, however, may not have very satisfactory activities and molecular weight distributions and improvement may thus be desired.